1. Field of the Invention
The present invention relates to devices for the production of composite panels under heat and pressure, and in particular to heated panel presses for the production of chipboard panels, fiber panels, and the like, presses of this type being either single-layer or multi-layer presses and having one stationary press spar and an opposing, movable press spar, with a heated pressure plate on each spar.
2. Description of the Prior Art
Heated panel presses of the above-mentioned type are notorious for their problems encountered with respect to the maintenance of the necessary plane-parallelism of the heated pressure plates during repeating pressing operation, as the pressure plates are alternatively heated and cooled.
These problems are related to the fact that the pressure plates, which are rather large in surface, are heated to considerable temperatures and that the uneven heat distribution within the structure of each press spar leads to thermal stress and subsequent distortions, with the result that the initial geometric flatness of the pressure plates is destroyed.
Various approaches have been suggested in the past for a correction of this problem, one such proposal being that the supporting structure of the press spars should be heated, too. Another approach suggests tha the spars should be cooled and that a heat barrier between the pressure plates and the press spar should be installed, in order to prevent any heat transmission.
Known prior art panel presses featuring such heat barriers have either a cooling grid and a cooling plate, or an insulating layer, against which the pressure plates are supported. However, the many machined surfaces necessary in such a structure, coupled with the corrosion encountered on these surfaces and with the effect of cumulative tolerances, to which must be added a comparatively rapid partial wear of the insulating layers, leads to the result that, even with a pressure plate machined to perfect flatness, no satisfactory pressing performance, under maintenance of the necessary plane-parallelism over a substantial length of time, is obtainable. The basic construction of such a press is disclosed, for example, in U.S. Pat. No. 3,594,867. Upper and lower press spars with insulated pressure plates are shown in U.S. Pat. No. 3,685,932 and 3,775,033.
Part of the reasons for the above problem relate to the tendency of the insulating layers to absorb humidity, thereby swelling and contracting, which adds to the simultaneous expansion and contraction caused by changes in temperature, thereby producing a friction effect between the pressure plates and the insulating layer, so that the latter is subjected to rapid abrasion and premature destruction. Furthermore, the compressibility of the insulating layers under pressure was found to change, as a result of the absorption of humidity by these layers.
A further shortcoming of the known prior art insulating layers for these pressure plates relates to their lack of dimensional stability, because of their inadequate long-term resistance to elevated temperatures, and to their inadequate compression resistance. The end result of such uneven wear and dimensional distortion of the insulating plates is that their thickness is unevenly affected and that the working surface of the pressure plates consequently are distorted so as to loose their required flatness. The resulting work product is a composite pressed panel of uneven thickness, necessitating either reworking of the panel, or even complete rejection and loss of the end product.
Prior art solutions using the approach of compensatory heating or cooling of the entire press spar structure have the shortcoming that such a structure becomes rather complex and costly, necessitating complicated temperature control devices. A further shortcoming relates to the fact that considerable time is necessary for the initial heating of such a structure, until a state is reached in which all parts of the press spar are evenly heated.